There are several basic magnetic circuits that are used in magnetic drum separators that include permanent magnets, electromagnets, or a combination of the two types. Each type of magnetic circuit has its own drawbacks and advantages. Electromagnetic elements warm up during operation which reduces their effectiveness over time and further limit the distance over which they can operate.
There are also inefficiencies associated with the coil wrapped around a steel core in electromagnets that cannot be avoided which creates areas of lower strength magnetic fields near each end of the rotating drum assembly. In contrast, permanent magnets can be manufactured that take up the entire width of the drum shell giving a more even magnetic field across the entire width of the drum assembly.
When electromagnets heat up, they lose typically 30 percent of their ampere turns (ampere turns generate magnetic field). This corresponds to nearly 30 percent of the magnetic field they generate. Permanent magnets are less affected by heat increases than electromagnets. Permanent magnets also do not have the power requirements that electromagnets need to generate magnetic fields.
One of the biggest drawbacks with permanent magnets is there is no way to turn them off which makes maintenance, repair, and even routine cleaning difficult if any magnetic metal equipment is used. Therefore permanent magnets with lower strength than are ideal have been used in the past. What is proposed is a system in which stronger permanent magnets can be used in a drum separator such that the permanent magnet elements can be moved away from the surface of the rotating drum element to reduce the strength of the magnetic field as needed for maintenance, repair, and routine cleaning. In addition to making permanent magnet drum separators easier to maintain, making the rare earth permanent magnet pickup magnets moveable within the drum separator also allows the operator to “fine-tune” the strength of the magnetic field to vary the grade of the product stream in the ferrous and the non-ferrous fraction as required for the particular application.